Bushing insulator



July 23, 1940. H1.. RORDEN 2,209,003

BUSHING INSULATOR Filed Jan. 5l, 1939 Fl .3 INVENTOR G] L, HaroldLRorden BY vf/y'! lw` 'l ATTORNEY `Patented July 23, 1940 UNITEDl STATESPATENT OFFICE RUSHING INSULATOR Harold L. Rorden, Barberton,

The Ohio Brass Company,

Ohio, assignor to Mansfield, Ohio, a

2 Claims.

This invention relates to electric insulator bushings and has for one ofits objects the provision of a bushing in which the electrostatic stressand consequently the voltage gradient is more effectively distributedthan has heretofore been possible.

A further object of the invention is to provide a bushing insulator thatWill withstand higher voltages for a given size than similar insulatorsheretofore manufactured.

A further object of the invention is to raise the corona voltage andradio disturbance voltage of bushing insulators.

A further object of the invention is to provide a device of the classnamed which shall be of improved construction and operation.

Other objects and advantages will appear from the following description.

The invention is exemplified by the combination and arrangement of partsshown in the accompanying drawing and described in the followingspeciflcation, and it is more particularly pointed out in the appendedclaims.

In the drawing:

Figs. 1 and 2 are diagrams illustrating typical electrostatic fieldsabout an insulator bushing.

Fig. 3 is a Somewhat diagrammatic fragmentary sectional view of abushing insulator having the present invention applied thereto.

One of the most difficult problems in the design of insulators is tosecure an effective distribution of the voltage gradient between theelectrodes. In most insulator designs a much greater amount ofdielectric material is employed than would be necessary if it werepossible to secure a uniform voltage gradient between electrodes, due toconcentration of the electrostatic field at one or more points. Thisproblem is particularly difcult in the design of bushing insulatorsbecause of the peculiar shape and relation of the electrodes involved.

Fig. 1 illustrates the problem involved in a bushing insulator in whichfigure the numeral I designates the conductor, and the numeral IIdesignates the ring forming the opening through which the conductorextends. It Will be seen from this gure that the electrode I0 is a long,narrow one while the electrode II is substantially uni-planar. The linesI2Y in the diagram diagrammatically represent the electrostatic fluxbetween the electrodes I0 and I I.` It will be seen that the lines offorce are widely distributed along the electrode I Il, but areconcentrated in the plane of the electrode II so that where a conductorpasses through a ring shaped opening as is the casein a bushinginsulator, and where there is a uniform dielectric about the electrodes,there is a great concentration ofelectrostatic flux between the twoelectrodes in the plane of the ring so that the dielectric immediatelysur- 5 rounding the conductor in this plane is subjected to a muchhigher stress than is the dielectric in other portions of the f'leldand, when the voltage between the two electrodes is sufficiently high,breakdown will take place along the lines of greatest concentration ofstress; that is, between the ring and the conductor in the plane of thering. In order to counteract this tendency to some extent, it hasheretofore been the practice to insert a tubular baille between theconductor IIJ and the ring or bushing flange II,

as indicated at I3 in Fig. 2. This baille is made of some material suchas porcelain which has a higher dielectric strength than air and theexpedient increases the breakdown value of a bushing for a given spacingbetween the electrodes. Available dielectrics for this purpose, however,all have a higher specific inductive capacity or dielectric constantthan does air so that the insertion of a dielectric baille between theconductor and the flange transfers a large portion of the dielectricstress from the interposed baille to the surrounding air and produces aconcentration of stress along the outer surface of the baffle extendingfrom the flange as indicated at I4 in Fig. 2. This gives rise to coronastreamers and, where sufficient voltage is applied, to flashoverdischarges emanating from the flange and extending along the outersurface of the dielectric baille.

In Fig. 3 there is shown a bushing insulator having a central conductorI5 surrounded by dielectric baffles I 6 and I1. 'I'he usual flange I8engages a shoulder I 9 on the outer baille I'I. The usual dielectriccone 20 engages the upper face of the flange I 8 and supports the upperterminal 2I of the bushing, which in this case is in the form of anexpansion chamber for the oil within the bushing. It is usual to fillbushings of this kind with a dielectric liquid such as transil oil tosuppress internal discharges. The lower end of the bushing is shown assubmerged in insulating liquid indicated at 22. In bushings of this kindthere has heretofore been a tendency for streamers to start along theouter surface 50 of the cone 2D from the upper edge of the flange I8 andalso down along the outer surface of the baille I'I toward the lowerbushing terminal 23. These streamers result from the concentration ofelectrostatic lines of force in the air, the con- 55 centration beingdue to the fact that the air has a lower dielectric constant than theporcelain bames and the oil within the bushing.

In order to avoid high concentration of electrostatic stress within theoil zone immediately surrounding the conductor, it has been foundadvantageous to coat the inner surface of the inner baffle i8 with aconducting coating 28 which may or may not be electrically connected tothe conductor. This prevents overstress of the oil immediately adjacentthe conductor. To prevent discharge streamers on the outer surface ofthe baille il within the apparatus housing upon which the bushing ismounted, it has heretofore been the practice to employ a conductingsleeve known as a ground sleeve, extending downwardly from the ange itto below the surface level of the oil in the apparatus housing.` Such asleeve is ,o

shown at 25 in Fig. 3 of the drawing and may be a metallized coatingover the surface of the bame il. One method of applying such a coatingis described in the patent to Ray Higgins, No. 2,119,989 dated June '7,1938 and assigned to The Ohio Brass Company. As explained in thatpatent, the metal may be sprayed over a prepared surface indicated at 26and disposed between the dielectric member and the outer coating 25. Thecoatings 25 and 26 as shown in the drawing, are

relatively much thicker than they are in actual construction, it beingimpossible to show these coatings in their true proportions in a drawingof the scale used.

The ground sleeve as heretofore used, however, does not overcome theconcentration of electrostatic stress illustrated in Fig. 2, but merelycarries the point of greatest stress downwardly from the lower edge ofthe flange I8 to a point below the insulating oil. Since the oil has ahigher dielectric constant than air, the stress at the lo wer end of thecoating 25 will not be as great as it would be at the lower edge of theflange if there were no ground sleeve and if the ange is not covered byoil. Furthermore, the oil has a higher dielectric strength than has air,and consequently, the use of a ground sleeve which projects below theoil counteracts to some extent the tendency to discharge from the flangewithin the apparatus housing. It does not, however, entirely remove thetendency to discharge at the lower end of the sleeve, and if the voltageis high there may be formation of corona at the lower edge of the sleeveeven when the end of the sleeve is coveredwith oil. Moreover, the groundsleeve does not affect the high stress in the air at the upper edge ofthe flange so that in bushings as heretofore constructed there has beena considerable overstress and tendency to form corona discharge from theupper edge of the flange.

I have found that a much more uniformly distributed ileld, andconsequently a much more uniform distribution of voltage gradient can beobtained by the use of a high resistance conducting coating disposed onproperly selected portions of the dielectric members of the insulator.Any suitable high resistance coating which will permanently adhere tothe ceramic surface may be employed. 'Ihe resistance of this coating may`vary between rather Wide limits. One method of measuring the resistanceis by contacting spaced points on the surface by the electrodes of amegger. Measuring the resistance of the surface in this way with contactwires le in diameter and having hemispherical contact ends and with thewires spaced 1/2" apart, I have found that surfaces having a resistancebetween aaoaoos points 1/ apart of approximately 5,000 megohms, givevery satisfactory results. However. beneficial results can be obtainedwith surfaces in which the resistance may range from to 50,000 megohmswhen so measured. The resistance of the coating should be sullicient toreduce the voltage at the edge of the coating to a value at whichdischarge will not take place. The voltage at which discharge will takeplace, and consequently the amount of resistance necessary to preventdischarge will depend somewhat upon the nature of the dielectriccovering the edge of the coating. A greater reduction in voltage isnecessary where the edge of the coating is exposed to the atmospherethan is required where the edge of the coating-is covered with oil sinceoil has a greater puncture strength and a higher dielectric constantthan air. If the edge of the coating `is covered with wax or othermaterial having a high puncture strength and a high dielectric constant,the resistance of the coating can be less than where the edge is exposedin the atmosphere. and consequently the resistance of the coating neednot be so great where the terminal edge is thus covered.

One suitable high resistance coating for controlling the stressdistribution in bushing insulators is the base coating described inPatent No. 2,119,989 mentioned above. The coating shown at 26 in Fig. 3of the drawing may be such a coating, and where a coating of this natureis used, the metal forming the ground sleeve coating 25 may be sprayedon directly over the outer surface of the base coating 2S. The groundsleeve coating terminates slightly below the surface level of the oil inthe apparatus housing, but the high resistance conducting coatingextends beyond the termination 'of the ground sleeve coating for somedistance. The distance that the base coating extends beyond the groundsleeve coating' may be from l@ to 3 or more inches, depending upon thevoltage to which the insulator is subjected and the resistance of thecoating. Due to the fact that this base coating is slightly conducting,the charging current will follow the coated surface so that it willnotlbreak down the surrounding dielectric, and thus it will suppress anytendency for corona to form at the lower edge of the ground sleeve.Since the coating has a high resistance there, of course, will be avoltage gradient due to the flow of the charging current in this coatingso that the voltage will gradually decrease from the lower edge of theground sleeve coating to the lower edge of the base coating, thuspreventing a concentration of voltage at the lower edge of the groundsleeve and producing a distribution of the electrostatic ux along thehigh resistance conducting coating, as indicated in Fig. 3.

Concentration of electrostatic stress at the outer upper edge of theflange i8 may also be avoided by continuing the high resistanceconducting coating along the outer surface of the baille il to a pointabove the flange I8 as indicated at 2l in Fig. 3, and by coating thelower portion of the inner surface of the cone 20 as indicated at 28 inFig. 3. The high resistance conducting coatings 21 and 28 produce avoltage gradient along the surface on which they are deposited and thusproduce a distribution of electrostatic stress in the surroundingdielectric as indicated in Fig. 3 of the drawing. It will be seen that alarge proportion of the electrostatic lines of force instead ofemanating from the outer upper edge of the flange I3 and passing throughthe surrounding air, as shown in Fig. a, win 75 emanate from thecoatings 2l and 28 and extend through the bafiles so that they form anelectrostatic screen diverting the flux from the outer corner of theflange i8 where it Would produce a high stress in the surrounding airand consequent formation of corona. The coatings 2l' and 28 not only:form electrostatic screens for diverting the lines of force from thedanger points, but also produce voltage gradients which give adistribution of the electrostatic eld over a considerable area, thusavoiding concentration of the stress at any one point. Of course, a highresistance coating could be deposited on the outer surface of the cone2U extending upwardly from the iiange I8, and such a coating wouldproduce a voltage gradient tending to reduce the stress at the flange.There are certain advantages gained, however, by locating the resistancecoating within vthe dielectric member. One of these advantages is thatthe extremity of the coating is covered by the oil in the bushing, andanother is that the lines oi' force emanating from the coating must passthrough the porcelain shell before reaching the air, thus suppressingformation of corona along these lines.

It has been found in practice that where the electrostatic field about abushing insulator is distributed by high resistance conducting surfacesas described, a bushing oi a given size will withstand several times thevoltage without producing discharges that a similar bushing without suchcontrols can withstand.

I claim:

1. The combination with a conductor, an apparatus housing having anopening in the wall thereof through which said conductor extends andinsulating liquid Within said housing, of a tubular insulator separatingsaid conductor from said housing, a ground sleeve surrounding saidconductor but insulated therefrom and electrically connected with thewall of said housing and extending beneath the surface of said liquid,and means forming a high resistance conducting surface extending beneathsaid liquid from the termination of said ground sleeve for a distancebeyond the termination of said ground sleeve for distributing theelectrostatic flux emanating from said ground sleeve and preventinglashover of said bushing.

2. The combination with a housing for electrical apparatus having anopening in the wal thereof, of a conductor extending through saidopening, insulating liquid within said housing, a dielectric bailleinterposed between said conductor and the wall of said housingsurrounding said opening, said baie having a ground sleeve of conductingmaterial electrically connected with the wall of said housing andextending to a point beneath the surface level of the liquid in saidhousing, means forming a high resistance conducting surface extendingbeyond the termination of said ground sleeve and terminating beneath theoil in said housing for distributing the electrostatic ux and preventingconcentration of stress at the termination of said ground sleeve, atubular dielectric shell disposed outside of said housing and separatingsaid conductor from the wall of said housing, insulating liquidsurrounding said conductor within said shell and means forming a highresistance conducting surface extending beyond the wall of said housingin the opposite direction from said ground sleeve for diverting theelectrostatic flux from the air outside of said shell and for preventingoverstress adjacent the wall of said housingV surrounding saidconductor, said conducting surface terminating within the portion ofsaid shell lled with insulating liquid.

HAROLD L. RORDEN. (0

